Coil antennas used to provide and receive information using electromagnetic induction is constituted by a resonant circuit, and a resonant frequency is generally adjusted (tuned) to efficiently provide and receive information. Important parameters for determining a resonant frequency of a resonant circuit include self-inductance and capacitance of the resonant circuit. The resonant frequency is generally adjusted by changing values of these self-inductance and capacitance.
When a plurality of electrical circuits with self-inductance is placed close to each other, mutual inductance occurs. This mutual inductance is also an important parameter for efficiently providing and receiving information. In general, the mutual inductance is designed to have a value as large as possible in an assumed use environment, aiming to achieve strong communication coupling between the antennas at the transmitting and the receiving ends.
One application using a coil antenna is a non-contact information medium. A non-contact information medium has no contact with a reader/writer and therefore causes no contact failure, thus being available at a position away from the reader/writer. Non-contact approaches have been in increasing demands due to their features such as being resistant to stain, rain, and static electricity and their high security level.
A non-contact information medium gains operating power by electromagnetic induction, using an electric wave received from a reader/writer, and exchanges information with the reader/writer using an electric wave of a certain frequency. To this end, each of the non-contact information medium and the reader/writer has a built-in coil antenna for transmitting and receiving an electric wave of a certain frequency.
A conventional non-contact information medium basically includes: a coil forming an antenna section that receives electric power from the outside, and transmits and receives information; a capacitor forming a resonant circuit with this coil; and an integrated circuit (IC) chip that controls processing operation to be performed by the non-contact information medium. To operate such a non-contact information medium, the non-contact information medium is placed within a communication range near the reader/writer. This causes mutual interaction between the resonant circuit of the non-contact information medium and a resonant circuit of the reader/writer, thus producing induced current in the coil constituting the antenna section of the non-contact information medium. Using this induced current as operating power, the IC chip operates to transmit information to the reader/writer through the coil constituting the antenna section. In this way, by performing wireless communication, the non-contact information medium and the reader/writer transmit and receive information (see, for example, Patent Document 1).
When a plurality of non-contact information media is placed near a reader/writer, however, mutual interaction occurs not only between the non-contact information media and the reader/writer but also between the non-contact information media. In this case, the use environment is different from a use environment assumed when designed. This results in a change in resonant frequency of the non-contact information media, and reduces the effect of the mutual interaction with the reader/writer, often failing to perform communication.
As a countermeasure for such problems, in Patent Document 2, a chip coil (a lumped constant inductor) is provided in a portion of each coil of non-contact information media. Further, the mutual interaction between the non-contact information media is reduced, while the frequency of an electric wave used for communication is matched to the resonant frequency of the non-contact information media. In this way, the read performance is improved when a plurality of non-contact information media is placed near a reader/writer.
Patent Document 1: Japanese Patent Application Laid-open No. 2001-34725
Patent Document 2: Japanese Patent Application Laid-open No. 2006-67479